Fixes #164

Adds a convenience method so that users of the API
don't have to import a lot in order to add an annotation.
I found the need for this API while instrumenting a client.
It avoids userland imports of:
* datetime
* attributes.Attributes
* trace.time_event.Annotation
* trace.time_event.TimeEvent

abstracting the above work, to no imports and just:
```python
span.add_annotation('Here is my annotation', key1='Foo', key2=12)
```

I believe the other implementations have a
method that helps add annotations.

Language|MethodName|Resource
---|---|---
Go|Span.Annotate|https://godoc.org/go.opencensus.io/trace#Span.Annotate
Java|Span.addAnnotation|https://www.javadoc.io/doc/io.opencensus/opencensus-api/0.12.2

Address review feedback from Angela.